1. Field of the Invention
This invention generally relates to impact printers, and, in particular, to printing hammer assemblies for use in impact printers.
2. Description of the Prior Art
Impact printers are well known in the art. There are two major categories in impact printers: one category includes line printers which print all of the characters in one printing line at the same time so that printing is carried out line by line and the other category includes serial printers which print characters serially one after another along a printing line. In the latter category, one typical example is a wheel printer which uses a print wheel, sometimes called "daisy wheel", comprised of a hub, a plurality of spokes extending radially from the hub and various types respectively provided at the free ends of the spokes, and an impact hammer for applying an impact force to a selected one of the types upon locating the selected type by rotating the print wheel at a predetermined printing position.
Printing hammer assemblies are employed in various impact hammers, whether serial or line, and it generally includes a printing hammer having an impact surface for applying an impact force to a selected type and a driving solenoid which drives to move the printing hammer forward electromagnetically when energized. In such printing hammer assemblies, their printing hammers are moved back and forth at high frequencies between the home or retracted position and the advanced position where the printing hammer makes contact with a selected type thereby applying an impact force to the selected type to form an imprint on recording paper. Since the impact conditions vary depending upon from where the printing hammer starts its forward movement when driven by the driving coil, it is important that the printing hammer resides at a predetermined home or retracted position at all times before being driven to move for the next printing operation. If there is a relatively large clearance for the home position of the printing hammer, the stroke of movement of the printing hammer will vary thereby causing to deteriorate the quality of printed characters. Thus, it is important to insure that the printing hammer can be returned precisely to the same home position at all times.
One example of a prior art printing hammer assembly applied to a wheel printer using a printing wheel is illustrated in FIG. 1. As shown, the printing hammer assembly includes a printing hammer 1 having a hammer shaft 2 at its forward end and an armature 3 at its backward end, and the printing hammer 1 is supported by a pair of front and rear bearings 4 and 5 so as to be slidably movable in a reciprocating manner linearly. The assembly also includes a front yoke 6 which has a front cylindrical section 6a to which the front bearing 4 is fixedly mounted. A driving solenoid 7 is provided as wound around a spool 26 which, in turn, is fixedly mounted as housed in the front yoke 6. The solenoid 7 is so disposed that it can interact with the armature 3 electromagnetically when energized.
A rear yoke 8 is tightly fitted into the rear end of the front yoke 6 and the rear bearing 5 is fixedly attached to the rear yoke 8, and, thus, the front and rear yokes 6 and 8 are so combined to establish a magnetic circuit. Also provided in the assembly is a cover 9 which encloses the front and rear yokes 6 and 8. A rubber damper 10 is disposed as supported by the cover 9 at the location opposite to the rear end of the printing hammer 1, and a metal plate 12, which is a thin rigid member such as a washer, is fixedly attached to the front end surface of the rubber damper 10 thereby defining a retracted end position P by the front end surface of the metal plate 12. As shown in FIG. 1, since a coil spring 11 is provided as extended between the armature 3 of the printing hammer 1 and the front bearing 4, the printing hammer 1 is located at its home or retracted position with its rear end surface abutting against the metal plate 12 when the driving coil 7 is in deenergized state. Also provided in the assembly of FIG. 1 is a protector 32 as fixedly attached at the mouth of the front cylindrical section 6a.
In operation, when the driving solenoid 7 is energized, there is produced a magnetic flux passing through the front yoke 6, printing hammer 1 and rear yoke 8 so that the printing hammer 1 is electromagentically driven to move forward against the force of the compression spring 11 as indicated by the arrow A. Thus, the front end, defined as impact surface, of the printing hammer 1 applies an impact force to a selected type 36 of a print wheel 34 located at a predetermined printing position, and, therefore, the type 36 is strongly pressed against recording paper 42 placed around a platen roller 40 with an ink ribbon 38 sandwiched therebetween. Thus, an imprint of the type 36 comes to be formed on the paper 42. Upon deenergization of the solenoid 7, the printing hammer 1 returns to its home position as receiving the recovery force from the spring 1 until its rear end surface hits the washer 12.
The rubber damper 10 is provided for the purpose of absorbing the shock energy of the printing hammer 1 when it returns to its home position with the aid of the recovery force of the spring 11 in order to prevent the printing hammer 1 from rebounding, and it is typically comprised of a low elastic rubber material. A main objective of provision of the washer 12 is to prevent the rear end surface of the printing hammer 1 from being adhered to the front end surface of the rubber damper 10 because the rear end surface of the printing hammer 1 is normally kept pressed against the rubber damper 10 under the force of the spring 11.
However, in the prior art printing hammer assembly as described above, accuracy in positioning the printing hammer 1 at its home position is relatively poor due to several causes. For example, it is rather difficult to precisely control the dimensional accuracy of the rubber damper 10 such as molding accuracy and positioning accuracy. Further, the rubber damper 10 tends to deform due to aging and other environmental conditions thereby causing to shift the retracted position P. Moreover, since rubber is severely affected by deterioration in durability due to aging, performance tends to fluctuate and thus operation is not trustworthy. Such irregularities in retracted position of the printing hammer will produce irregularities in the level and timing of an impact force to be applied to a selected type, and, therefore, resulting imprints will be poor in quality. Such a tendency will be made more noticeable if printing speed is desired to be increased.